1. Field of the Invention
This invention relates to a field decision circuit for deciding the fields of video signals in an interlaced scanning system.
2. Description of the Background
In a display of a video signal used in the interlaced scanning system (hereinafter, such a video signal is referred to as "an interlaced video signal"), one frame consists of an odd field in which odd-number scanning lines are selected and an even field in which even-number scanning lines are selected. Therefore, field information to determine whether the current field is the odd field or even field is essential for a display apparatus for performing interlaced scanning display. Conventionally, there have been no independent field decision circuits, and the output from a field judgment circuit which judges the odd and even fields of an interlaced video signal has been used as field information in other circuits or units which require such information.
However, such a judgment circuit often mal-functions. The factors which cause a judgment circuit to malfunction are related to an interlaced video signal or synchronizing signal supplied to a field judgment circuit which includes a noise or variation in the timing of a synchronizing signal which appears during a reproduction operation of a videotape recorder. There arises no problem unless such noise is generated or such variation does occur. However, it is difficult to completely prevent the malfunction of a field judgment circuit under weak broadcasting waves or during a special reproduction operation of a videotape recorder in which the rotary system is improperly adjusted.
In a flat display apparatus, the influence of the discrimination of video signal fields on the image quality are growing because of their driving characteristics. Particularly, a large-sized liquid crystal display apparatus uses field information as location deciding information for reproducing a video signal on its screen, and also as information for alternating the polarity of voltages applied to pixels. Therefore, field judgment errors may degrade the display quality or cause an irreversible change of the optical state of the liquid crystal (i.e., a breakdown of the display apparatus).